"Witnesses said the group basically bombed its way into the prison, creating a scene of complete carnage. In the ensuing chaos, between 100 and 200 prisoners escaped," she said.

Investigation launched

The government said an investigation into the attack had begun.

Hadijha Aminu, a local prison official, said: "There was a jail break last night. From reports available to us, a large number of gunmen attacked the Koton-Karifi prison around 7pm and threw explosives at the gate and opened fire on our wardens."

She said a shootout then erupted.

"One of our men was killed and the gunmen overpowered the wardens and broke into the cells, freeing inmates," Aminu said.

Odeyemi, the Nigeria Prisons Service spokesman, said the motives of the attackers were unclear.

"It might be that some of the armed robbers are trying to free the armed robbers there awaiting trial.''

Odeyemi said he did not know if the prison held any members of Boko Haram, the radical Islamist group which has been plaguing the country with violence over the last year.

Al Jazeera's Ndege said: "The belief is that they were trying to free Boko Haram suspects; there were about 20 Boko Haram suspects awaiting trial inside the prison."

Boko Haram launched a similar prison break in Bauchi state in September 2010 that freed about 700 inmates.

The style of the attack on Wednesday, which apparently used explosives and heavy gunfire, matched thatprevious assault by Boko Haram.

Members of the group have been blamed for killing at least 286 people this year alone, according to an Associated Press news agency count.

Violent campaign

Boko Haram's violence comes as part of a campaign which its leader, Abubakar Shekau, says is aimed at avenging Muslim deaths, freeing imprisoned members and pushing for strict Islamic law across multi-ethnic Nigeria, a nation of more than 160 million people.

No group immediately claimed responsibility for Thursday's prison attack, and authorities said no suspects were immediately in mind.

If Boko Haram carried out Wednesday night's attack, it would be the farthest strike south the group has made. Nigeria is largely split between a Christian south and a Muslim north.

Most of Boko Haram's previous attacks have taken place in the north.

Nigeria's prisons remain overcrowded and understaffed, with the majority of those imprisoned awaiting trials for years.

A 2007 study by Amnesty International, the UK-based rights organisation, called the system "appalling", with children remaining locked up with their parents and guards routinely bribed by inmates.

Despite pledges by the government to reform the system, it remains largely the same today.

Our correspondent said Thursday's prison attack would be another public relations setback for the government.

"Nigeria's security services are under tremendous pressure to control Boko Haram and this prison break will be a huge embarrassment," Ndege said.

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